1. Field of the Invention
The present invention relates to sheet-like articles having an oriented polymer continuous phase and polymer microbeads dispersed therein that are at least partially bordered by voids. These articles have unique properties of texture, opaqueness, whiteness in the absence of colorants, and generally good physical properties such as thermal stability, durability, low density and the like. They are especially suitable for paper substitutes. More particularly, the invention relates to oriented polymer sheet like materials, the orientation of which requires lower physical forces, while at the same time providing sheet-like articles having lower density, higher opacity and lightness and lower yellowness.
2. Description of Related Art
U.S. Pat. No. 4,994,312 issued Feb. 19, 1991 to L. K. Maier et al. and assigned to the same assignee as the present invention relates to shaped articles including films and sheets from a matrix orientable polymer containing cross-linked microbeads which upon orientation of the matrix polymer causes microvoids. The microbeads are prepared utilizing VAZO 52, 2,2'-azobis-(2,4-dimethylpentane nitrile), a monomer soluble initiator sold by DuPont, as the initiator for the polyaddition reaction.
Free radical initiators are generally of two classes. See, for example, "The Selection and Use of Free Radical Initiators" by Chester S. Sheppard and Vasanth Kamath published in Polymer Engineering and Science, July 1979, Volume 19, No. 9. These two classes are peroxide compounds and azo compounds.